Fate of pesticides in bottom sediments during dredging and disposal cycle.

Recent concerns for protection of the environment have dictated that many of the polluted bottom sediments dredged from harbours waterways be deposited within dyked containment areas. In the case of a hopper dredge, which is frequently used for this purpose, these sediments are mixed with large quantities of ambient river water and pumped into the disposal area in the form of a slurry with about 15% solids content. Then, the solids settle out of suspension and the excess water usually flows over a weir back into the harbour or river; on rare occasions a filtering system of sone type is used in place of the weir. Accordingly the effectiveness of this dredging and disposal operation depends on the extent to which the pollutants are associated with the solids that are retained within the containment area. Many previous studies have indicated the polluted dredgings may contain a variety of natural wastes, fertilizers, pesticides, detergents, metals, oil grease, and many other by-products from industrial processes. Because of the high degree of pollution in many cases, micro-organisms that normally degrade the pollutants and purify the water are unable to survive; moreover, even if they were able to survive, some industrial chemicals, particularly many of the pesticides, are intracable. This study is directed toward; (i) evaluating the extent to which various pesticides are associated with the solid portion of the dredgings, and (ii) advancing some possible correlations and explanations for the observed behaviour. (A)